1. Field of the Invention
The present invention relates to a device and method for suppressing coherent jamming signals received in a scattered signal receiving system and more particularly to such a device and method using a pair of physically spaced antennas and a phase and amplitude adjusting circuit to null coherent jamming signals.
2. Background of the Invention
The use of radio frequency signals deflected and/or defracted from various layers of the ionosphere for long distance radio communication systems is well known. Such communication systems are further widely used in military applications.
In a military environment, it is also common for an enemy to use jamming signals directed at receiving antennas for the purpose of disrupting the operation of the communication systems. One method used to overcome jamming signals is to use frequency hopping systems. In this type of system the carrier frequency of the transmitter and receiver are switched rapidly and in synchronism over a multiplicity of frequencies. Such systems are effective to the extent that they demand that an enemy produce jamming signals at most if not all of the frequencies used in the frequency hopping scheme. However, even in the best case, jamming signals will be effective against portions of the transmission thereby significantly reducing system performance. It is therefore desirable to provide a communication system having a capability of distinguishing between communication signals and jamming signals to thereby reduce the effect of jamming signals even when they are being produced at a communication system operating frequency.
Ionospheric and/or tropospheric scattered (reflected) radio frequency signals, because they traverse a multiplicity of transmission path lengths between transmitter and receiver, arrive at different points in space as a non-coherent signal. In contrast, a jamming signal propagates from a single source and will arrive at a receiving antenna as a substantially coherent signal. Even if several jamming signals are being generated at a single source or from a plurality of sources, each of the jamming signals will retain this characteristic of being a coherent signal at the receiving antenna. This characteristic difference between a scattered signal communication signal and an interference signal provides the basis of the present invention.